In recent years, fiber laser systems including (i) a plurality of fiber lasers, (ii) an output combiner for combining laser light outputted by each of the plurality of fiber lasers, and (iii) a output section for outputting laser light from the output combiner to outside the laser system (for example, toward a processing target object) have been attracting attention as a way to achieve an output power of several kW (kilowatts). Light outputted by each of the plurality of fiber lasers propagates through a single-mode fiber or a quasi-single-mode fiber to the output combiner. Light outputted by the output combiner propagates through a multi-mode fiber to the output section. Such a fiber laser system is used mainly in the field of material processing.
With the above fiber laser system, there is a risk that light reflected by the processing target object will be reflected back to the fiber laser system and cause a malfunction thereof.
A reason for such malfunctioning of the fiber laser system is the occurrence of stimulated Raman scattering (hereinafter also referred to as “SRS”) in a passage through which laser light propagates in the fiber laser system. SRS can be considered a process of power conversion from laser light to Stokes light. It is known that an increase in the conversion efficiency (Raman gain) thereof corresponds to a greater likelihood of the occurrence of Stokes light vibration. As a result, laser light generation in each fiber laser becomes unstable, and malfunctions can therefore occur in each fiber laser (see Patent Literature 1).